Crate handlebars

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Handlebars is a modern and extensible templating solution originally created in the JavaScript world. It’s used by many popular frameworks like Ember.js and Chaplin. It’s also ported to some other platforms such as Java.

And this is handlebars Rust implementation, designed for general purpose text generation.

Quick Start

use std::collections::BTreeMap;
use handlebars::Handlebars;

fn main() {
  // create the handlebars registry
  let mut handlebars = Handlebars::new();

  // register the template. The template string will be verified and compiled.
  let source = "hello {{world}}";
  assert!(handlebars.register_template_string("t1", source).is_ok());

  // Prepare some data.
  // The data type should implements `serde::Serialize`
  let mut data = BTreeMap::new();
  data.insert("world".to_string(), "世界!".to_string());
  assert_eq!(handlebars.render("t1", &data).unwrap(), "hello 世界!");

In this example, we created a template registry and registered a template named t1. Then we rendered a BTreeMap with an entry of key world, the result is just what we expected.

I recommend you to walk through handlebars.js’ intro page if you are not quite familiar with the template language itself.


Handlebars is a real-world templating system that you can use to build your application without pain.

Isolation of Rust and HTML

This library doesn’t attempt to use some macro magic to allow you to write your template within your rust code. I admit that it’s fun to do that but it doesn’t fit real-world use cases.

Limited but essential control structures built-in

Only essential control directives if and each are built-in. This prevents you from putting too much application logic into your template.

Extensible helper system

Helper is the control system of handlebars language. In the original JavaScript version, you can implement your own helper with JavaScript.

Handlebars-rust offers similar mechanism that custom helper can be defined with rust function, or rhai script.

The built-in helpers like if and each were written with these helper APIs and the APIs are fully available to developers.

Auto-reload in dev mode

By turning on dev_mode, handlebars auto reloads any template and scripts that loaded from files or directory. This can be handy for template development.

Template inheritance

Every time I look into a templating system, I will investigate its support for template inheritance.

Template include is not sufficient for template reuse. In most cases you will need a skeleton of page as parent (header, footer, etc.), and embed your page into this parent.

You can find a real example of template inheritance in examples/ and templates used by this file.

Strict mode

Handlebars, the language designed to work with JavaScript, has no strict restriction on accessing nonexistent fields or indexes. It generates empty strings for such cases. However, in Rust we want to be a little stricter sometimes.

By enabling strict_mode on handlebars:


You will get a RenderError when accessing fields that do not exist.


Compatibility with original JavaScript version

This implementation is not fully compatible with the original JavaScript version.

First of all, mustache blocks are not supported. I suggest you to use #if and #each for the same functionality.

There are some other minor features missing:

  • Chained else #12

Feel free to file an issue on github if you find missing features.


As a static typed language, it’s a little verbose to use handlebars. Handlebars templating language is designed against JSON data type. In rust, we will convert user’s structs, vectors or maps into Serde-Json’s Value type in order to use in templates. You have to make sure your data implements the Serialize trait from the Serde project.


Template Creation and Registration

Templates are created from Strings and registered to Handlebars with a name.

use handlebars::Handlebars;

  let mut handlebars = Handlebars::new();
  let source = "hello {{world}}";

  assert!(handlebars.register_template_string("t1", source).is_ok())

On registration, the template is parsed, compiled and cached in the registry. So further usage will benefit from the one-time work. Also features like include, inheritance that involves template reference requires you to register those template first with a name so the registry can find it.

If you template is small or just to experiment, you can use render_template API without registration.

use handlebars::Handlebars;
use std::collections::BTreeMap;

  let mut handlebars = Handlebars::new();
  let source = "hello {{world}}";

  let mut data = BTreeMap::new();
  data.insert("world".to_string(), "世界!".to_string());
  assert_eq!(handlebars.render_template(source, &data)?, "hello 世界!".to_owned());
Additional features for loading template from
  • Feature dir_source enables template loading register_templates_directory from given directory.
  • Feature rust-embed enables template loading register_embed_templates from embedded resources in rust struct generated with RustEmbed.

Rendering Something

Since handlebars is originally based on JavaScript type system. It supports dynamic features like duck-typing, truthy/falsey values. But for a static language like Rust, this is a little difficult. As a solution, we are using the serde_json::value::Value internally for data rendering.

That means, if you want to render something, you have to ensure the data type implements the serde::Serialize trait. Most rust internal types already have that trait. Use #derive[Serialize] for your types to generate default implementation.

You can use default render function to render a template into String. From 0.9, there’s render_to_write to render text into anything of std::io::Write.

use handlebars::Handlebars;

struct Person {
  name: String,
  age: i16,

  let source = "Hello, {{name}}";

  let mut handlebars = Handlebars::new();
  assert!(handlebars.register_template_string("hello", source).is_ok());

  let data = Person {
      name: "Ning Sun".to_string(),
      age: 27
  assert_eq!(handlebars.render("hello", &data)?, "Hello, Ning Sun".to_owned());

Or if you don’t need the template to be cached or referenced by other ones, you can simply render it without registering.

use handlebars::Handlebars;

  let source = "Hello, {{name}}";

  let mut handlebars = Handlebars::new();

  let data = Person {
      name: "Ning Sun".to_string(),
      age: 27
  assert_eq!(handlebars.render_template("Hello, {{name}}", &data)?,
      "Hello, Ning Sun".to_owned());

As per the handlebars spec, output using {{expression}} is escaped by default (to be precise, the characters &"<> are replaced by their respective html / xml entities). However, since the use cases of a rust template engine are probably a bit more diverse than those of a JavaScript one, this implementation allows the user to supply a custom escape function to be used instead. For more information see the EscapeFn type and Handlebars::register_escape_fn() method. In particular, no_escape() can be used as the escape function if no escaping at all should be performed.

Custom Helper

Handlebars is nothing without helpers. You can also create your own helpers with rust. Helpers in handlebars-rust are custom struct implements the HelperDef trait, concretely, the call function. For your convenience, most of stateless helpers can be implemented as bare functions.

use std::io::Write;
use handlebars::{Handlebars, HelperDef, RenderContext, Helper, Context, JsonRender, HelperResult, Output, RenderError};

// implement by a structure impls HelperDef
#[derive(Clone, Copy)]
struct SimpleHelper;

impl HelperDef for SimpleHelper {
  fn call<'reg: 'rc, 'rc>(&self, h: &Helper, _: &Handlebars, _: &Context, rc: &mut RenderContext, out: &mut dyn Output) -> HelperResult {
    let param = h.param(0).unwrap();

    out.write("1st helper: ")?;

// implement via bare function
fn another_simple_helper (h: &Helper, _: &Handlebars, _: &Context, rc: &mut RenderContext, out: &mut dyn Output) -> HelperResult {
    let param = h.param(0).unwrap();

    out.write("2nd helper: ")?;

  let mut handlebars = Handlebars::new();
  handlebars.register_helper("simple-helper", Box::new(SimpleHelper));
  handlebars.register_helper("another-simple-helper", Box::new(another_simple_helper));
  // via closure
      Box::new(|h: &Helper, r: &Handlebars, _: &Context, rc: &mut RenderContext, out: &mut dyn Output| -> HelperResult {
          let param = h.param(0).ok_or(RenderError::new("param not found"))?;

          out.write("3rd helper: ")?;

  let tpl = "{{simple-helper 1}}\n{{another-simple-helper 2}}\n{{closure-helper 3}}";
  assert_eq!(handlebars.render_template(tpl, &())?,
      "1st helper: 1\n2nd helper: 2\n3rd helper: 3".to_owned());

Data available to helper can be found in Helper. And there are more examples in HelperDef page.

You can learn more about helpers by looking into source code of built-in helpers.

Script Helper

Like our JavaScript counterparts, handlebars allows user to define simple helpers with a scripting language, rhai. This can be enabled by turning on script_helper feature flag.

A sample script:

{{percent 0.34 label="%"}}
// percent.rhai
// get first parameter from `params` array
let value = params[0];
// get key  value pair `label` from `hash` map
let label = hash["label"];

// compute the final string presentation
(value * 100).to_string() + label

A runnable example can be find in the repo.

Built-in Helpers
  • {{{{raw}}}} ... {{{{/raw}}}} escape handlebars expression within the block
  • {{#if ...}} ... {{else}} ... {{/if}} if-else block (See the handlebarjs documentation on how to use this helper.)
  • {{#unless ...}} ... {{else}} .. {{/unless}} if-not-else block (See the handlebarjs documentation on how to use this helper.)
  • {{#each ...}} ... {{/each}} iterates over an array or object. Handlebars-rust doesn’t support mustache iteration syntax so use each instead. (See the handlebarjs documentation on how to use this helper.)
  • {{#with ...}} ... {{/with}} change current context. Similar to {{#each}}, used for replace corresponding mustache syntax. (See the handlebarjs documentation on how to use this helper.)
  • {{lookup ... ...}} get value from array by @index or @key (See the handlebarjs documentation on how to use this helper.)
  • {{> ...}} include template by its name
  • {{log ...}} log value with rust logger, default level: INFO. Currently you cannot change the level.
  • Boolean helpers that can be used in if as subexpression, for example {{#if (gt 2 1)}} ...:
    • eq
    • ne
    • gt
    • gte
    • lt
    • lte
    • and
    • or
    • not
  • {{len ...}} returns length of array/object/string

Template inheritance

Handlebars.js’ partial system is fully supported in this implementation. Check example for details.


pub use self::template::Template;



Macro that allows you to quickly define a handlebars helper by passing a name and a closure.


A data structure holds contextual data for current block scope.
A map holds block parameters. The parameter can be either a value or a reference
The context wrap data you render on your templates.
Render-time Decorator data when using in a decorator definition
The single entry point of your Handlebars templates
Render-time Helper data when using in a helper definition
Json wrapper that holds the Json value and reference path information
The context of a render call
Error when rendering data on template.
Error on parsing template.


Represents the Json path in templates.
A JSON wrapper designed for handlebars internal use case


Decorator Definition
Evaluate decorator
Helper Definition
Render Json data with default format
The Output API.
Render trait


The default escape fn replaces the characters &"<> with the equivalent html / xml entities.
EscapeFn that does not change anything. Useful when using in a non-html environment.
Convert any serializable data into Serde Json type

Type Definitions

This type represents an escape fn, that is a function whose purpose it is to escape potentially problematic characters in a string.
A type alias for Result<(), RenderError>